This invention relates to a hydraulic machine, i.e. a motor or pump, having at least one working chamber of cyclically changing volume and valve means to control the connection of the or each chamber to low- and high-pressure manifolds. The invention also relates to a method of operating the machine.
The invention has particular reference to machines where the at least one working chamber comprises a cylinder in which a piston is arranged to reciprocate, but its use with at least one chamber delimited by a flexible diaphragm or a rotary piston is not ruled out.
With most hydraulic machines the fluid chambers undergo cyclical variations in volume following a sinusoidal function. It is known from EP-A-0361927 that a chamber can be left to idle by holding an electromagnetically actuated valve, between the working chamber and the low-pressure source, in the open condition. Thus the output is varied through the through the action of first filling each working chamber with liquid, then deciding whether to reject the liquid back to the low-pressure source or to pump it at pressure to the output manifold. Pumping the liquid back to the low-pressure source means that a very small amount of power needs to be expended, during the time that a working chamber is idle, whilst still allowing the working chambers to become productive with a minimum latency period.
It has become common practice to regulate the flow of piston pumps by closing the intake means part way through the stroke such that the entrained air in the liquid expands and forms bubbles in the liquid during the closed expansion, which occurs below atmospheric pressure. On compression, this volume of air is first compressed before the liquid component of the cylinder reaches the pressure required to open the face-seating non-return valve connected to the high-pressure manifold. It is only once pressure and consequently density of the liquid in the working chamber has increased sufficiently and the valve has opened that delivery to the high-pressure manifold can commence. If the piston has significant velocity when the gas bubble volume nears zero, then the rapid pressure rise, that accompanies this change of compliance, creates an impulsive rise in stress throughout the machine, which results in both audible and fluid-borne noise.